Remove deployment Slack notifications from the deployer pipeline

On #1489 (closed), Slack notifications were added to the coordinated pipeline, slack notifications can also be sent from the deployer pipeline. Which notification will be used depends on how a deployment is triggered:

• Standard way: Deployments are triggered from the coordinated pipeline in release tools, in this case, the slack notifications will be sent from the coordinated pipeline.
• Alternative way: Deployments are triggered using ChatOps, this option is used if:
  • Production is blocked due to incidents
  • We want to speed up the deployment of a package
  • We want to deploy a specific package into one of our environments (rollbacks)

For the alternative way, notifications will be sent from the deployer pipeline

Having two sources for the notifications increases maintainability and makes it hard to reason about, it'd be great if we could have one single source of truth for notifications.

Proposal

Make the coordinated pipeline to be the single source of truth for slack notifications and remove the notifications logic from the deployer pipeline.